The present invention relates to an optical information recording and reproducing apparatus and, more particularly, to an optical information recording and reproducing apparatus having a tracking servo system for positioning a light spot, which is to be irradiated on tracks on a disk-shaped optical recording medium.
There is known in the art an optical information recording and reproducing apparatus which is constructed such that a disk-shaped optical recording medium having a plurality of concentric or spiral tracks for recording information is rotated and irradiated with a laser beam focused by an optical system so that the information recorded on the medium may be read out in terms of the changes in the reflected light. For this apparatus, the tracking operation to position of the light spot to follow the tracks is indispensible. There is disclosed in U.S. Pat. No. 4,363,116, for example, the apparatus of the type in which the recording medium is formed in advance with such grooved guide tracks for the aforementioned tracking operation as have optical characteristics different from those of their surrounding regions so that the information may be recorded on those guide grooves. The light beam emitted from its light source irradiates the aforementioned guide tracks via beam directing means such as a galvanomirror arranged on the optical path, and the reflected light is partially inputted to two-split optical detectors, for example, so that the aforementioned beam directing means is controlled by the differential signal of that optical detector functioning as a tracking error signal. Since the light spot is enabled to follow the tracks on which no information has been recorded, according to the aforementioned tracking system with the guide grooves, this system is especially useful for an apparatus which has the function of additionally writing new information at an arbitrary position on a track.
In the optical information recording and reproducing apparatus of the this kind, however, the light spot formed on the disk surface may have a diameter as small as 1.6 microns. As a result, when the light spot scans a defective position on a track, the defect adversely affects the light spot so as to act upon the beam directing means as a false tracking error signal. In this case, the beam directing means operates to restore the tracking error signal level to zero. If, however, the error signal generated when the defective position is scanned is a strong signal, the light spot strays out of the normal track position to the adjacent track. If this phenomenon occurs during recording, data may be recorded on the wrong track or lost completely.
To prevent erratic tracking during recording or playback, a system has been proposed which is equipped with means for detecting that the tracking error signal exceeds a predetermined value, for example, so that the laser emission may be interrupted as soon as a malfunction is detected by the detecting means (as is disclosed in Japanese Patent Laid-Open No. 57-1501440). With this system, tracking is always interrupted at a defective position on a track; consequently, the defective track cannot be accessed. This makes it necessary to conduct strict inspections upon the recording medium produced so as to reject the medium as a defective if the medium has a defect which will cause erratic tracking.
If erratic tracking is detected during the recording or playback, tracking is interrupted so as to leave the rest of that track unusable thereby reducing the area of the usable recording surface.